Flywheels provide an economical and environmentally-friendly means for storing energy. Flywheels store energy by converting electrical energy into kinetic energy of the rotating mass of a flywheel rotor turning at a relatively high velocity (i.e., 20,000 to 30,000 rpm). A motor/generator may be coupled to the flywheel to accelerate the flywheel rotor to the relatively high velocity. The motor/generator may be coupled to an electrical power system. For example, the motor/generator may be coupled to a utility power grid. During periods of low demand for electricity, the electric motor/generator may draw electricity from the utility power grid to accelerate the flywheel rotor and convert the electricity into kinetic energy of the rotating flywheel rotor mass. Once the flywheel rotor reaches the desired velocity, electricity may be provided to the motor/generator on an intermittent or as-needed basis to maintain the flywheel rotor at the desired velocity. During periods of high demand for electricity, the flywheel rotor mass may be used to rotate the motor/generator to convert the kinetic energy of the flywheel rotor back into electricity which may be distributed to the utility power grid.
The efficiency of a flywheel energy storage system can be significantly improved by housing the flywheel in a vacuum chamber. The vacuum chamber may improve the efficiency of the flywheel energy storage system by minimizing thermal losses in bearings that support the flywheel. The bearings may include passive magnetic bearings provided in combination with permanent magnets and a high temperature superconductor magnet system. The permanent magnets provide a lifting force for suspending the flywheel in position. The superconductor magnet system may stabilize the position of the flywheel. In order to maintain the superconductor magnet system at the relatively low temperatures required (e.g. 77° Kelvin or colder), a liquid nitrogen cooling system may be included with the flywheel energy storage system to circulate liquid nitrogen through superconductors. Unfortunately, liquid nitrogen cooling systems impose a significant weight penalty and require regular maintenance and servicing in order to maintain the liquid nitrogen at the necessary levels.
In an attempt to avoid the weight and maintenance penalties associated with liquid nitrogen cooling systems, self-contained cryocoolers may be implemented in the flywheel energy storage system to cool the superconductors. Unfortunately, at the relatively low temperatures required for operating the superconductor magnet system, air molecules in the vacuum chamber may transfer a significant amount of heat between the superconductors. The heat transferred by the air molecules may exceed the ability of the cryocoolers to maintain the superconductor at the low operating temperatures. In addition, cryocoolers may consume relatively large amounts of power which may reduce the overall efficiency of the flywheel energy storage system.
The vacuum chamber may also improve the efficiency of the flywheel energy storage system by minimizing aerodynamic drag or frictional losses that may occur when gas molecules (e.g., air molecules) in the vacuum chamber come into contact with the flywheel rotor outer surface moving at a relatively high velocity. Over a relatively short period of time, the friction between the air molecules and the flywheel rotor outer surface may result in significant heating of the flywheel to an extent that the structural integrity of the flywheel may be compromised. Attempts to minimize the quantity of air molecules within the vacuum chamber and reduce the vacuum pressure include mounting several different types of vacuum pumps to the vacuum chamber. For example, a transfer pump may be mounted to the vacuum chamber to urge gas molecules toward an outlet of the transfer pump whereupon the gas molecules may be discharged to the outside environment. Unfortunately, such transfer pumps may result in localized areas within the vacuum chamber that have a relatively high vacuum such as near the pump outlet while remaining areas within the vacuum chamber have a reduced vacuum level (i.e. relatively higher pressure) such as at the flywheel rotor outer surface.
As can be seen, there exists a need in the art for a system for providing an ultra-high vacuum in a vacuum chamber which has minimal power requirements. In addition, there exists a need in the art for a system and method for providing an ultra-high vacuum in a vacuum chamber which has a relatively low system weight in order to improve the power density of the flywheel energy storage system such that overall system efficiency is increased. Additionally, there exists a need in the art for a system and method for providing an ultra-high vacuum in a vacuum chamber which provides for a uniform vacuum throughout the vacuum chamber such that aerodynamic drag is minimized at the flywheel rotor outer surface.